The term microscopy is employed where a surface is imaged with radiation of a same energy. Where radiation of different or varying energies is used the term spectroscopy is generally employed. Dual purpose instruments are generally designated as microscopes even when they perform spectroscopic investigation as well.
Spectroscopic analysis of surfaces at atomic scales is desirable for a number of reasons, including the identification and characterization of surface impurities in semiconductor, superconductive and other structures.
In U.S. Pat. No. 4,343,993, Aug. 10, 1982, Binnig et al. describe a vacuum electron tunneling effect that is utilized to form a scanning tunneling microscope. In an ultra-high vacuum at cryogenic temperature, a fine tip is raster scanned across the surface of a conducting sample at a distance of a few Angstroms. The vertical separation between the tip and sample surface is automatically controlled so as to maintain constant a measured variable which is proportional to the tunnel resistance, such as tunneling current.
In a journal article entitled "Atomic Force Microscope", Physical Review Letters, Vol. 56, No. 9, G. Binnig et al. at pages 930-933 described an atomic force microscope that is said to combine the principles of the scanning tunneling microscope and a stylus profilometer.
In U.S. Pat. No. 4,724,318, Feb. 9, 1988, Binnig describes an atomic force microscope wherein a sharp point is brought near enough to the surface of a sample to be investigated that forces occurring between the atoms at the apex of the point and those at the surface cause a spring-like cantilever to deflect. The cantilever forms one electrode of a tunneling microscope, the other electrode being a sharp tip. The deflection of the cantilever provokes a variation of the tunnel current, the variation being used to generate a correction signal which can be employed to control the distance between the point and the sample. In certain modes of operation, either the sample or the cantilever may be excited to oscillate in a z-direction. If the oscillation is at the resonance frequency of the cantilever, the resolution is enhanced.
In U.S. Pat. No. 4,747,698 Wickramasinghe et al. describe a scanning thermal profiler wherein a fine scanning tip is heated to a steady state temperature at a location remote from the structure to be investigated. Thereupon, the scanning tip is moved to a position proximate to, but spaced from the structure. At the proximate position, the temperature variation from the steady state temperature is detected. The scanning tip is scanned across the surface structure with the temperature variation maintained constant. Piezo electric drivers move the scanning tip both transversely of, and parallel to, the surface structure. Feedback control assures the proper transverse positioning of the scanning tip and voltages thereby generated replicate the surface structure to be investigated.
In a journal article entitled "Atomic Force Microscope-Force Mapping and Profiling on a Sub 100-A Scale", J. Appl. Phys. 61 (10), 15 May 1987, Y. Martin et al., at pages 4623-4729 describe a technique for accurate measurement of the force between a tip and a material, as a function of the spacing between the tip and the material surface. The technique features a tip that is vibrated at close proximity to a surface in conjunction with optical heterodyne detection to accurately measure the vibration of the tip. The technique enables the measurement of tip displacements over large distances and over a wide range of frequencies, which is a major advantage over the previous methods. The technique is applicable to non-contact profiling of electronic components on scales varying from tens of microns to a few tens of angstroms. A second application is described wherein material sensing and surface profiling are achieved simultaneously.
In a journal article entitled "High-resolution capacitance measurement and potentiometry by force microscopy", Appl. Phys. Lett. 52 (13), Y. Martin, D. W. Abraham and H. K. Wickramasinghe at pages 1103-1105 describe an atomic force microscope employed for potentiometry and for imaging surface dielectric properties through the detection of electrostatic forces.
The electron tunneling effect is shown to be applicable to spectroscopic analysis is a journal article entitled "Tunneling Spectroscopy", B. J. Nelissen and H. van Kemper, Journal of Molecular Structure, 173 (1988) at page 141-156. This article describes the use of the Scanning Tunneling Microscope as a spectroscopic probe. These authors note that spectroscopic methods use energetic probes, usually photons, to gain desired information. They further note that for spectroscopy in conducting solids the use of photons is not an obvious choice, since the electrons inside the solid can be used as spectroscopic probes.
In a journal article "Photothermal Modulation of the Gap Distance in Scanning Tunneling Microscopy", Appl. Phys. Lett. 49 (3), 21 July 1986, by Nabil M. Amer, Andrew Skumanich and Dean Ripple at pages 137-139 describe the use of the photothermal effect to modulate the gap distance in a tunneling microscope. In this approach, optical heating induces the expansion and buckling of a laser-illuminated sample surface. The surface displacement can be modulated over a wide frequency range, and the height (typically .sctn. 1 Angstrom) can be varied by changing the illumination intensity and modulation frequency. The method is said to provide an alternative means for performing tunneling spectroscopy.
As is apparent the Scanning Tunneling Microscope (STM) and the Atomic Force Microscope (AFM) have provided an efficient and accurate means to perform the observation of atomic features on surfaces However, such prior art techniques have not overcome the problem of providing an efficient and accurate means to perform spectroscopy on the atomic and/or molecular scale, although certain attempts have yielded some limited results, namely voltage spectroscopy in STM, "peak force detection" spectroscopy with the AFM, temperature spectroscopy with the Thermal Profiler, and Auger spectroscopy with a Field emission microscope.
It is thus an object of the invention to provide apparatus and method for performing spectroscopy at atomic scales.
It is another object of the invention to provide method and apparatus for practicing Atomic Photo-Absorption Force Microscopy (APAFM) that beneficially combines both atomic resolution and spectroscopy for use in wide range of analytical applications.